Improved confinement characteristics of pellet fuelled discharges on JT-60

Abstract

Improved energy confinement for additionally heated JT-60 limiter and lower x-point discharges has been obtained with hydrogen pellet injection. The energy confinement time is enhanced by up to 40% over that of usual gas fuelled discharges at a medium NB or NB + ICRF heating power level of < 15 MW, for which a strongly peaked electron density profile with ne(0)/⟨ne⟩ < 5 and ne(0) < 2.0 × 1020 m−3 is sustained within 0.5 s after a series of pellet injections. The achieved values of the fusion product ne(0)τE(a)Te(0) (<6.8 × 1019 m−3·s·keV) at Ip = 2.5 MA exceed those obtained previously on JT-60 at Ip = 3.1 MA. The improved discharges are characterized by a pressure profile that peaks strongly around the magnetic axis (inside the sawtooth inversion radius) and degrades when a large sawtooth recovers or when the pressure gradient reaches a critical value. The peakedness depends strongly on the penetration length of the pellets. In lower x-point discharges, the improvement is distinct in the low q (q < 3) regime, probably because of suppression of the sawtooth activity.

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“…In this discharge, the sawtooth repetition time in the pre-pellet portion is 0.08-0.1 s and the sawtooth activity is completely suppressed within ~0.8 s after pellet injection. For lower x-point discharges, the peaked portion of SX emission profiles is completely flattened by the sawtooth activity [1]. For pellet fuelled limiter discharges, the sawtooth activity does not seem to be of the full reconnection type, and the released central energy at the sawtooth crash is small (see Section 3).…”

“…For pellet fuelled discharges in JT-60, highly peaked electron pressure profiles have been obtained [1]. Figure 1 shows profiles of the electron density rie(r) and the electron temperature T e (r) measured 0.8 s after pellet injection for a typical 3.1 MA limiter discharge (toroidal magnetic field B, = 4.8 T, safety factor at the plasma surface q(a) = 2.3, absorbed heating power Pabs -1 7 .…”

“…In JT-60, hydrogen pellets were injected into hydrogen discharges with Ohmic heating (OH), neutral beam injection (NBI) [1,2], and heating in the lower hybrid range of frequencies (LHRF) and/or the ion cyclotron range of frequencies (ICRF). The energy confinement time T E was enhanced by up to 40% relative to the usual gas fuelled discharges at medium NBI or NBI+ICRF heating powers of -5 -2 0 MW, for which a strongly peaked pressure profile was sustained for 0.5-1.0 s after a series of pellet injections [1][2][3].…”

“…The energy confinement time T E was enhanced by up to 40% relative to the usual gas fuelled discharges at medium NBI or NBI+ICRF heating powers of -5 -2 0 MW, for which a strongly peaked pressure profile was sustained for 0.5-1.0 s after a series of pellet injections [1][2][3]. One of the interesting characteristics of the improvement in confinement is its strong dependence on the central MHD behaviour, namely the sawtooth activity.…”

“…The basic scenario of pellet injection, the plasma equilibrium configuration and the diagnostic systems are presented in Ref. [1]. In contrast to experiments on JET [11], where pellets were injected into sawtooth-free discharges (q(0) > 1, with q(0) the safety factor at the plasma centre), we injected the pellets into discharges with q(0) < 1 and obtained the characteristics discussed in the following section.…”